The invention relates generally to strapping tools, and more particularly to improved plastic strap welding tools useable in combination with strap tensioning tools and methods therefor.
In load packaging operations, it is known generally to weld overlapping portions of plastic strap tensioned about the load. U.S. Pat. No. 3,564,033, issued Apr. 4, 1972, entitled "Strap Tensioning and Sealing Tool" and assigned commonly herewith, for example, discloses a pneumatically operated strapping tool that forms a friction-fused joint, or weld, by vibrating contacting interfacial surfaces of overlapping plastic strap portions. The strapping tool includes a clamping member for anchoring a free strap end portion, and a rotatable feed wheel cooperating with an anvil foot to engage a feed strap portion, which is tensioned about the load. A clutch engagably couples the feed wheel to a pneumatic motor, which also vibrates a jaw that welds the overlapping strap portions. The motor stalls and tensioning stops at a specified strap tension, and a pneumatically actuated ram disengages the motor from the feed wheel and engages a feed wheel brake, which maintains feed wheel tension on the tensioned strap during welding. The actuated ram also moves the vibrating jaw into frictional engagement with the overlapping strap portions. As the vibrating jaw moves toward the overlapping strap portions, a shearing edge severs the upper strap, and the overlapping strap portions are subsequently welded together. Thereafter, pressure is maintained on the welded overlapping strap portions for a cool down period to complete the weld.
U.S. Pat. No. 5,380,393, issued Jan. 10, 1995, entitled "Hand Strapping Tool", also assigned commonly herewith and incorporated herein by reference, discloses a strapping tool having a pneumatic circuit for automatically controlling tool operation, including the timing and duration of strap welding and the cool down period. A pneumatically actuated ram extended after strap tensioning pivots a cam that moves a vibrating welding plate into contact with overlapping strap portions to form a weld. Vibration of the welding plate terminates after a time period controlled by the accumulation of air pressure in a chamber. Thereafter, air bled from a cylinder counter-pivots the cam to move the welding plate away from the welded strap portions after a cool down period.
Prior art pneumatic strapping tools require many system components that increase the size and weight of the tool, and increase costs related to tool manufacturing, operation and maintenance. The tool of U.S. Pat. No. 3,564,033, for example, requires a pneumatically actuated ram for disengaging the air motor from the feed wheel, engaging the feed wheel brake, and moving the vibrating jaw into engagement with the overlapping strap portions. U.S. Pat. No. 5,380,393 requires pneumatic rams for moving the welding plate into contact with the overlapping strap portions, and a combination of cylinders, chambers and valves for controlling the timing and duration of various tool operations. Although the pneumatic circuit of U.S. Pat. No. 5,380,393 automates many strapping operations performed manually in prior art tools, control and timing of the various tool operations is relatively imprecise, producing inconsistent strap tension and weld results.
Prior art strapping tools, including those discussed above, also have generally a relatively long base plate disposed between the strap and the load during strap tensioning and welding. Thereafter, the base plate is removed from between the tensioned strap and load. The relatively long base plate, however, has a tendency to create slack in the tensioned strap after its removal, particularly in applications where the load is relatively small or shaped irregularly. The loss in strap tension depends generally on the size of the base portion and on the size of the load, and in many cases results in inadequately tensioned strap.
The present invention is drawn toward advancements in the art of strapping tools generally, and more particularly to strapping tools for forming friction-fused joints, or welds, by vibrating contacting interfacial surfaces of overlapping strap portions, especially plastic strap portions.
It is an object of the invention to provide novel strap welding tools and methods therefor that overcome problems in the prior art, and novel strap welding tools that are economical to manufacture, operate and maintain.
It is also an object of the invention to provide novel strap welding tools, and more particularly plastic strap welding tools, and methods therefor that eliminate control and timing inconsistencies inherent in prior art pneumatically operated strapping tools by controlling various tool operations and especially the welding operation with cam members, which are preferably rotated by a common drive shaft.
It is a more particular object of the invention to provide novel strap welding tools and methods therefor having a first cam member for moving a sealing gripper toward a first support member to engage and retain tensioned strap disposed therebetween, a second cam member for enabling a vibrator motor to vibrate a welding pad, and a third cam member for moving the welding pad toward a second support member to engage and weld overlapping strap portions.
It is another object of the invention to provide novel strap welding tools and methods therefor having relatively small base plates to reduce loss of strap tension upon removal of the tool from strap tensioned about a load, particularly strap tensioned about relatively small and irregularly shaped loads.
It is another more particular object of the invention to provide novel strap welding tools and methods therefor comprising base plates having the first support member on a leading end portion thereof, the second support member on a second portion thereof, and a third support member cooperating with a tensioning gripper movable to engage and retain a free strap end portion during strap tensioning on a trailing end portion of the base plate, the second portion of the base plate intermediate the leading and trailing end portions thereof, and preferably the leading and trailing end portions slope downwardly away from the second portion.
It is a further object of the invention to provide novel strap welding tools and methods therefor that reduce column strength of a strap portion engagable by the vibrating welding pad by deforming the strap portion, and more particularly by bending the strap portion proximate the welding pad, preferably forming a gap between the overlapping strap portions.
These and other objects, aspects, features and advantages of the present invention will become more fully apparent upon careful consideration of the following Detailed Description of the Invention and the accompanying Drawings, which may be disproportionate for ease of understanding, wherein like structure and steps are referenced generally by corresponding numerals and indicators.